[Brian R.]

It sounds like your engine runs richer when you unhook the HO2 sensor (running in open-loop mode). This would indicate that whatever is causing the stumble is making your car run lean, since it is fixed by unhooking the HO2 sensor.

Running lean can be caused by many things, partially plugged injectors, poor fuel pressure from a bad pump, or plugged fuel filter, etc.

A sensor problem could also cause this, such as a dirty MAF or bad MAP or IAT sensor, ECT sensor, etc. See the post in the FAQ about the EFI feedback loop and the sensors which feedback to the ECM in a OBDI engine. One of them may be out of spec.